In room A, person A is dead. In room B, person B is dead. Each room has windows which are shut and cannot be manipulated from the outside, and each room has doors which are shut and locked. The two closed room deaths are related to each other in some fashion.

Ah, I gotcha. Alright. For putting up with that, I'll give you a bigger share of red.

At least one window in each room faced a window in the other room. Persons A and B saw each other prior to death, including int the moments prior to their deaths. Persons A and B could not confirm each others' death,
as whichever one managed to live a few seconds longer was too preoccupied with dying to take note of the other person.

Again, I am new here and I don't know if these guesses are too brute-forcey or not. Let's find out together.

* The murder of person A could not have occurred in the manner it did, if room A had no windows.* The murder of person A could not have occurred in the manner it did, if room A had no doors.* The murder of person A could not have occurred in the manner it did, if room A had no vents.* The murder of person A could not have occurred in the manner it did, if walls, ceiling, floor and entrances (windows, doors, vents) together accounted for all of the exterior of each room. (in reference to the statement in >>20768 )
* The murder of person A could not have occurred in the manner it did, if person A had become mute after the entrances to room A closed.* The murder of person A could not have occurred in the manner it did, if person A had become deaf after the entrances to room A closed.* The murder of person A could not have occurred in the manner it did, if person A had become blind after the entrances to room A closed.* The murder of person A could not have occurred in the manner it did, if person A had become armless after the entrances to room A closed.* The murder of person A could not have occurred in the manner it did, if person A had become legless after the entrances to room A closed.

* (same with A and B switched)

EDIT:

* Room A and B share at least one door.* Room A and B share at least one window.* Room A and B have connected vents.

* The murder of person A could not have occurred in the manner it did, if room B had no windows.* The murder of person A could not have occurred in the manner it did, if room B had no doors.* The murder of person A could not have occurred in the manner it did, if room B had no vents.* The murder of person A could not have occurred in the manner it did, if person B had become mute after the entrances to room A closed.* The murder of person A could not have occurred in the manner it did, if person B had become deaf after the entrances to room A closed.* The murder of person A could not have occurred in the manner it did, if person B had become blind after the entrances to room A closed.* The murder of person A could not have occurred in the manner it did, if person B had become armless after the entrances to room A closed.* The murder of person A could not have occurred in the manner it did, if person B had become legless after the entrances to room A closed.

The murders of persons A and B could not have occurred in the manner it did, if walls, ceiling, floor and entrances (windows, doors, vents) together accounted for all of the exterior of each room.

Rooms A and B do not officially share doors, windows, or vents, although they may have doors, windows, or vents touching the doors, windows, or vents touching the doors, windows, or vents of the other room. (hope that makes sense)

Persons A and B were aware that their death was a possibility when they entered the rooms. Neither wanted to die. Person B took more steps to guard against it than Person A, but both accepted the risk of death.

* Person A killed person B by interacting with the vent (eg pushing or rotating it) in room A, which is part of a mechanism that also connects to room B, either directly (eg through a string) or indirectly (eg by throwing a rock at a switch in the exterior of room B).* (same, but window instead of vent)* (same, but door instead of vent)

Persons A and B only interacted with the doors in their respective rooms to open them from the outside, close them from the inside, and lock them from the inside. None of these actions had an effect on the other person's room.

Persons A and B never interacted with the windows of their rooms.

Person A interacted with the vent of Room A to increase airflow into room A, but this act did not have an effect on the other person's room. Person B never interacted with the vent of room B

The murder weapon that killed Persons A and B are the same type, but different individual items. As an example: if they had been killed by handguns, the type of weapon would be "handgun", but they would have each been shot by a different gun, and the model of gun could be different.

The murder weapons for Persons A and B are not in the rooms of the person they killed (A in A, B in B). The definition of murder weapon is item which was required in order to cause a fatality. Again, going back to the gun example, the gun is considered the murder weapon, even though the bullet does the actual damage.

On a related note, a long-overdue red: The cause of death for Persons A and B were the same, though the manner of death was different. Going on a poison example, two people could have the cause of death be "poison." However, one person could have bumped into a contact poison that caused his throat to swell up, while the other could have been injected with poison that liquefied his organs. Thereby, both have the cause of death of "poison," but have very different manners and symptoms of death. This overwrites the second red in >>20760

Well, I suppose they could somehow induce each other into suicide...maybe. Alright.

Their wounds are not directly self-inflicted. If you're going to try to pick at indirectly self-inflicted, you'll need to be rather clear on how exactly that worked in this case. I just realized that 'indirect self-infliction' could cover just about anything, including trying to fight off a mugger and getting stabbed, so I put in'directly'.

So Person A wanted to show this guy in the other room a knife trick, so he took a knife from his room. Person B also wanted to show A a knife trick, so he grabbed a knife that was in his room also.
So they looked at each other through the window, smiled, and attempted to do the knife trick, but they failed horribly and both knives went flying to the vents where upon contact transformed into little tiny particles of knife which flew to the other room.
When the knives arrived on the other side, they materialized again and went flying to the people in the room.
Person B screamed in surprise, making the knife which was flying onto him enter his body through his mouth, messing up his insides.
In the other room, the other knife had hit Person A in the head, killing him instantly.
Person B died after realizing he had eaten a knife.

* Person A killed person B, by inducing him to ingest something orally, which created internal injury.* Person A killed person B, by making the results of person B ingesting something orally different from what person B thought they would be, creating internal injury.

* Person A killed person B by crushing him with room A.* Person A killed person B by making the results of person B trying to crush him with room B different from what person B expected.* (same with A and B switched)

* Persons A and B were in cars. They drove their cars into each other's, with murderous intent! Both died as result of the collision, through different injuries inflicted upon them by their own cars and its contents.

* The vents have nothing to do with anything, besides that cars tend to have them.

Correct. Persons A and B, enemies who wanted each other dead, drove their cars at each other in a game of chicken, each assuming the other would swerve. Person B was wearing a seatbelt, Person A was not.
Person B died from a broken neck caused from the seatbelt being improperly positioned, and Person A was impaled on the steering column.
The cars ended up mashed together, but the interiors remained separate, and although greatly damaged, the windows did not fall apart. Thereby, two closed rooms.